I've noticed this bug too in a diferent situation, but only part of the text is visible in the editable area of the INPUT. You can click on that text and even scroll through the entire string of text using the keyboard or dragging the mouse. I've found the fix to be the same: Give the positioned DIV a width. Link to the test page I created is below:
http://help.newdigitalgroup.com/bugs/Firefox/textarea_and_input_overlap/